BIOMEDICAL APPLICATIONS V: INFLUENCE OF CARBON NANOTUBES IN NEURONAL LIVING NETWORKS - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 154

The neuronal growth was visualised by SEM, and viability was determined
using calcein staining. Neurons were shown to grow on the SWNT-PEI
ilms with neurite outgrowth and branching that are intermediate to those
observed for neuronal growth on PEI and as-produced MWNTs (AP-MWNTs).
Indeed, the neurite branching on SWNT-PEI was enhanced by comparison
with AP-MWNTs, while it was comparable to PEI. The number of neurites
and growth cones was similar to that of AP-MWNTs, and the neurite lengths
were intermediate to those of neurons grown on pristine MWNTs and PEI
(Scheme 6.2b). Hence, the growth parameters were found to be sensitive
to the nature of the substrate, in particular the surface charge, since the
effects of the SWNT-PEI composite on neuronal growth characteristics are
intermediate to those observed when using pristine MWNTs and PEI. This
behaviour can be explained by the reduced positive charge of PEI, which is
proportional to the percentage of SWNTs in the composite. It should be noted
that the results obtained in this study with SWNT-PEI were compared with
those obtained with pristine MWNTs, not with unmodiied SWNTs. Hence,
the authors pointed out that the structural parameters of CNTs, in particular
the diameter, could also contribute to the observed effects, in addition to the
contribution of PEI.
In summary, neurite outgrowth and branching could be controlled by
varying the ratio of SWNTs and PEI in the graft copolymer. This composite
could be implemented in building scaffolds for the formation of neuronal
circuits to develop neural prostheses.
Haddon et al. also investigated the possibility of using water-soluble
SWNTs as substrate for neuronal growth. 14 The SWNTs soluble in water were
found to induce an increase of neurite length, while a decrease of the number
of neurites and growth cones was observed.
SWNTs were functionalised with either PABS 11 or polyethylene glycol (PEG,
n ≈ 13) 15 to form the corresponding graft copolymers (Scheme 6.1, paths B and
D). Each polymer imparted water solubility to the SWNTs. The methodology
used for the functionalisation was based on amidation of the COOH functions
located at the nanotube ends. Hippocampal neuronal cells were cultured
on the water-soluble SWNT substrates. The neurons accumulated the vital
stain calcein, which is indicative of cell viability and of the biocompatibility of
the functionalised SWNTs. The copolymers were found to modulate neurite
outgrowth by increasing their length, while reducing the number of neurites
and growth cones (Scheme 6.2c). Hence, the neurons treated with the water-
soluble SWNTs exhibited sparser, but longer neurites.
The authors suggested that water-soluble SWNTs may modulate
intracellular Ca 2+ homeostasis as Ca 2+ inlux is known to regulate neurite
elongation. Indeed, Ca 2+ channel blockers can cause at low concentrations a
simultaneous reduction of growth cone ilopodia and an increased elongation
of neurite. 8a The same modiications were induced in this study by the water-